In recent years, it has been desired to reduce the amount of carbon dioxide in order to cope with global warming. Hence, a non-aqueous electrolyte secondary battery having a small environmental burden has been used not only in a mobile device or the like but also in a power source device of an electrically driven vehicle such as a hybrid vehicle (HEV), an electric vehicle (EV), or a fuel cell vehicle. A lithium ion secondary battery directed to the application to electrically driven vehicles is desired to have a large capacity and a large area.
Hitherto, a carbon material such as graphite, which is advantageous in terms of charge and discharge cycle lifetime or cost, has been used for a negative electrode of a lithium ion secondary battery. However, the carbon material has a disadvantage that a theoretical charge and discharge capacity of 372 mAh/g or more, which is obtained from LiC6 as a compound introduced with maximum amount of lithium, cannot be obtained since the battery is charged and discharged by absorbing lithium ions into graphite crystals and desorbing the lithium ions therefrom in the negative electrode active material using the carbon material such as graphite. Thus, by using only the negative electrode active material using the carbon material such as graphite, it is difficult to obtain a capacity and energy density that are high enough to satisfy vehicle use on the practical level.
On the other hand, a battery using a material which is alloyed with Li in the negative electrode active material has improved energy density as compared with a battery using a carbon material such as graphite of the related art, and thus such a material is expected to be used as a material of the negative electrode active material for vehicle use. For example, a Si material absorbs and desorbs 3.75 mol of lithium ion per mol in charging and discharging and has a theoretical capacity of 3600 mAh/g in Li15Si4 (=Li3.75Si).
However, a lithium ion secondary battery using the material which is alloyed with Li in the negative electrode, particularly, using the Si material has large expansion-shrinkage in the negative electrode at the time of charging and discharging. For example, volumetric expansion of a graphite material in the case of absorbing Li ions is about 1.2 times. However, the Si material has a problem of a decrease in cycle lifetime of the electrode due to a large volumetric change of about 4 times, which is caused by transition from an amorphous state to a crystal state when Si is alloyed with Li. In addition, in the case of a Si negative electrode active material, the capacity has a trade-off relationship with cycle durability. Thus, there has been a problem in that it is difficult to increase the capacity and improve the cycle durability concurrently.
In order to solve the problems described above, for example, a negative electrode active material which contains an amorphous alloy having a composition represented by formula: SixMyAlz, for a lithium ion secondary battery is proposed (for example, see JP 2009-517850 A). Herein, in the formula, x, y, and z represent an atomic percentage value, x+y+z=100, x≥55, y<22, and z>0 are satisfied, and M represents a metal formed of at least one kind of Mn, Mo, Nb, W, Ta, Fe, Cu, Ti, V, Cr, Ni, Co, Zr, and Y. In the invention described in JP 2009-517850 A, it is stated in paragraph [0018] that, by minimizing a content of the metal M, not only a high capacity but also good cycle lifetime is exhibited. In addition, it is stated that, by mixing a graphite material having small expansion, good cycle lifetime is exhibited.